Capacitors formed within, or “embedded” in, multilayer circuit boards often are beneficially used in addition to or in place of discrete surface mount (SMT) capacitors. Discrete capacitors frequently introduce undesirable inductance from the vias and solder joints used to connect the discrete capacitors to the circuit. Discrete capacitors also consume the circuit board's surface area, preventing the connection of other surface devices. In contrast, embedded capacitors frequently exhibit less inductance than surface capacitors because of their proximity to the driver and receiver and due to their lack of solder joints and extra vias. Further, because they are embedded in inner layers of the circuit board, surface area is freed for use by other surface mount devices.
For all of their benefits over discrete capacitors, the capacitive values of embedded capacitors often are not as consistent as those of discrete capacitors. While discrete capacitors may be manufactured to have a predictable capacitance with slight variation, embedded capacitors often have unpredictable capacitances resulting from the circuit board manufacturing processes. This problem is compounded once the embedded capacitor is laminated inside the substrate of the circuit board because conventional capacitance tuning techniques typically cannot be employed. As a result, when a circuit board having an embedded capacitor with inadequate capacitance is detected, conventional techniques typically call for adjusting the nominal capacitance by adding a surface mount capacitor in parallel, thereby introducing the undesirable effects of discrete capacitors. Conventional techniques, however, do not provide for an efficient way to decrease the capacitance of an embedded capacitor having excessive capacitance in a laminated circuit board. For such modification to take place using conventional techniques, the discrete capacitor typically would be placed in series with the embedded and, as such, would necessitate the inclusion of connecting pads on the board that are shorted together. The short is usually provided using a discrete zero ohm or low ohm resistor which would introduce the undesirable effects of discrete components noted above. Consequently, the circuit board typically is either used in its present condition, often to ill effect, or it is discarded.
In view of the foregoing, it would be desirable to provide techniques for tuning embedded capacitors that overcome the above-described inadequacies and shortcomings in an efficient and cost effective manner.